Millimeters to Inches Converter
1 millimeter (mm) equals 0.03937007874 inches (in).
Quick answer
To convert millimeters to inches, divide by 25.4. One millimeter is about 0.03937 inches, and 25.4 mm is exactly one inch.
- 1 millimeter = 0.03937007874 inches
- 1 inch = 25.4 millimeters
- Formula: in = mm ÷ 25.4
- Quick estimate: divide by 25, then take off 1.6%: 100 mm ÷ 25 = 4, exact 3.94 in.
1 millimeter = 0.03937007874 inches
How to convert millimeters to inches
in = mm ÷ 25.4
To convert millimeters to inches, divide the number of millimeters by 25.4.
Example: 42 mm to in
- Start with the value in millimeters: 42 mm.
- Divide by 25.4: 42 ÷ 25.4 = 1.653543307.
- Result: 42 millimeters = 1.653543307 inches.
Going the other way, 1 inch = 25.4 millimeters, so to convert inches back to millimeters you multiply by the same number. See inches to millimeters.
Where the conversion factor comes from
Since the international yard and pound agreement of 1959, the inch has been defined as exactly 2.54 centimeters. Before that, the US and the UK used slightly different inches (the US inch was 2.540005 cm). The 1959 definition made the conversion exact, so 2.54 is not a rounded number: every inch-to-centimeter conversion is a multiplication by exactly 2.54. One inch is therefore exactly 25.4 millimeters.
Mental math: mm to in in your head
Divide by 25 (multiply by 4 and move the decimal point two places left), then reduce by about 1.6%. For example 50 mm: 50 × 4 = 200 → 2.00, minus a little = 1.97 inches.
Useful anchors: 3 mm ≈ ⅛ in, 6 mm ≈ ¼ in, 13 mm ≈ ½ in, 19 mm ≈ ¾ in.
When you need to convert millimeters to inches
- Matching metric and imperial drill bits, bolts and wrenches.
- Jewelry, watch and bead sizes.
- Rainfall: weather services report in mm, US gauges in inches.
- Camera lens focal lengths and film formats.
Common mistakes
- Swapping a metric wrench for an inch one: 13 mm (0.512 in) is close to ½ in (12.7 mm) but will round bolt heads.
- Dividing by 2.54 (that is for centimeters, not millimeters).
- Reading decimal inches as fractions: 0.25 in is ¼ in, but 0.3 in is not ⅓ in exactly (0.333).
Everyday millimeters in inches
| Example | Millimeters | Inches |
|---|---|---|
| Headphone jack | 3.5 mm | 0.1378 in |
| Pencil diameter | 7 mm | 0.27559 in |
| AA battery diameter | 14.5 mm | 0.57087 in |
| 35 mm film width | 35 mm | 1.378 in |
| A heavy rainfall day | 50 mm | 1.9685 in |
| Smartphone thickness | 8 mm | 0.31496 in |
Millimeters to inches conversion table
| Millimeters (mm) | Inches (in) |
|---|---|
| 0.1 mm | 0.003937007874 in |
| 0.2 mm | 0.007874015748 in |
| 0.3 mm | 0.01181102362 in |
| 0.4 mm | 0.0157480315 in |
| 0.5 mm | 0.01968503937 in |
| 0.6 mm | 0.02362204724 in |
| 0.7 mm | 0.02755905512 in |
| 0.8 mm | 0.03149606299 in |
| 0.9 mm | 0.03543307087 in |
| 1 mm | 0.03937007874 in |
| 1.5 mm | 0.05905511811 in |
| 2 mm | 0.07874015748 in |
| 2.5 mm | 0.09842519685 in |
| 3 mm | 0.1181102362 in |
| 3.5 mm | 0.1377952756 in |
| 4 mm | 0.157480315 in |
| 4.5 mm | 0.1771653543 in |
| 5 mm | 0.1968503937 in |
| 6 mm | 0.2362204724 in |
| 7 mm | 0.2755905512 in |
| 7.5 mm | 0.2952755906 in |
| 8 mm | 0.3149606299 in |
| 9 mm | 0.3543307087 in |
| 10 mm | 0.3937007874 in |
| 11 mm | 0.4330708661 in |
| 12 mm | 0.4724409449 in |
| 13 mm | 0.5118110236 in |
| 14 mm | 0.5511811024 in |
| 15 mm | 0.5905511811 in |
| 16 mm | 0.6299212598 in |
| 17 mm | 0.6692913386 in |
| 18 mm | 0.7086614173 in |
| 19 mm | 0.7480314961 in |
| 20 mm | 0.7874015748 in |
| 21 mm | 0.8267716535 in |
| 22 mm | 0.8661417323 in |
| 23 mm | 0.905511811 in |
| 24 mm | 0.9448818898 in |
| 25 mm | 0.9842519685 in |
| 26 mm | 1.023622047 in |
More mm to in values
- 27 mm
- 28 mm
- 29 mm
- 30 mm
- 31 mm
- 32 mm
- 33 mm
- 34 mm
- 35 mm
- 36 mm
- 37 mm
- 38 mm
- 39 mm
- 40 mm
- 41 mm
- 42 mm
- 43 mm
- 44 mm
- 45 mm
- 46 mm
- 47 mm
- 48 mm
- 49 mm
- 50 mm
- 51 mm
- 52 mm
- 53 mm
- 54 mm
- 55 mm
- 56 mm
- 57 mm
- 58 mm
- 59 mm
- 60 mm
- 61 mm
- 62 mm
- 63 mm
- 64 mm
- 65 mm
- 66 mm
- 67 mm
- 68 mm
- 69 mm
- 70 mm
- 71 mm
- 72 mm
- 73 mm
- 74 mm
- 75 mm
- 76 mm
- 77 mm
- 78 mm
- 79 mm
- 80 mm
- 81 mm
- 82 mm
- 83 mm
- 84 mm
- 85 mm
- 86 mm
- 87 mm
- 88 mm
- 89 mm
- 90 mm
- 91 mm
- 92 mm
- 93 mm
- 94 mm
- 95 mm
- 96 mm
- 97 mm
- 98 mm
- 99 mm
- 100 mm
- 110 mm
- 120 mm
- 130 mm
- 140 mm
- 150 mm
- 160 mm
- 170 mm
- 180 mm
- 190 mm
- 200 mm
- 210 mm
- 220 mm
- 230 mm
- 240 mm
- 250 mm
- 260 mm
- 270 mm
- 280 mm
- 290 mm
- 300 mm
- 310 mm
- 320 mm
- 330 mm
- 340 mm
- 350 mm
- 360 mm
- 370 mm
- 380 mm
- 390 mm
- 400 mm
- 410 mm
- 420 mm
- 430 mm
- 440 mm
- 450 mm
- 460 mm
- 470 mm
- 480 mm
- 490 mm
- 500 mm
- 600 mm
- 700 mm
- 800 mm
- 900 mm
- 1,000 mm
- 1,200 mm
- 1,500 mm
- 2,000 mm
- 2,500 mm
- 3,000 mm
- 4,000 mm
- 5,000 mm
- 10,000 mm
What is a millimeter?
The millimeter (also spelled millimetre) is a metric unit of length equal to one thousandth of a meter, or one tenth of a centimeter.
What is an inch?
The inch is a unit of length in the imperial and US customary systems equal to exactly 2.54 centimeters.
Convert millimeters to other units
Convert other units to inches
Frequently asked questions
How many inches are in a millimeter?
One millimeter is 0.03937007874 inches.
How many millimeters are in an inch?
One inch is 25.4 millimeters.
What is the formula to convert millimeters to inches?
in = mm ÷ 25.4.
What is 10 mm in inches as a fraction?
10 mm is 0.3937 in, just over ⅜ inch (0.375 in); the nearest 1/32 is 13/32 in.
How many mm are in an inch?
Exactly 25.4 millimeters.